This invention relates to an improved flexible shaft for transmitting torque.
A shaft used for transmitting torque, including a flexible shaft, frequently has square or other polygonal end parts, each end part being adapted to be received in a square or other polygonal cavity of a corresponding fitting. However, when such a shaft is rotated, play between the outside of the end part and the inside of the cavity fitting causes undesirable noise and vibration. In the case of a flexible shaft, the noise and vibration is accentuated due to flexing of the shaft as it is rotated.
Various arrangements have been proposed for coupling the shaft end part to the adjacent fitting cavity so as to reduce noise and vibration. See, for example, U.S. Pat. No. 4,344,304 to Eiche entitled Coupling For Mandrels Or The Like. However, such arrangements are relatively complicated and costly, and are not well suited to use with flexible shafts.
The assignee of the present application had believed that imparting a twist to the end of a flexible shaft would improve its noise and vibration characteristics. However, despite efforts by employees of the assignee and an independent consultant retained by the assignee, extending over a period of several years, prior to the invention described herein the efforts to produce such a shaft were not successful.
Accordingly, an object of the present invention is to provide a flexible shaft having an improved arrangement for coupling an end part of the shaft to a fitting.
As herein described, there is provided a flexible shaft having at least one end portion formed to have a helical noncircular shape. While any shape of noncircular cross-section may be used, a polygonal cross-sectional shape is preferred; a square cross-sectional shape being desirable, particularly in view of the large number of existing fittings having cavities of square cross-section.
When an end part of the helical noncircular end portion of the shaft is inserted into the cavity of the fitting, the helical end portion rotates, resulting in an interference fit between the end part and the fitting. This interference fit tightly retains the end part of the shaft within the cavity, resulting in reduced noise and vibration when the shaft is rotated.
The helical noncircular end portion of the flexible shaft is preferably formed by swaging, utilizing a die set having the desired helical noncircular configuration.